1. Field of the Invention
The invention relates to an optical signal-regenerating unit having an input for receiving an optical signal pulse series to be regenerated, which signal pulse series has a modulation period T and a wavelength .lambda..sub.d, and an output for supplying a regenerated signal pulse series, said unit comprising a pulsed laser for supplying a pulse series having a pulse period T.
2. Description of the Related Art
In optical transmission systems for long-distance information transport, an optical transmitter provided with a laser is used for converting a digital electric signal into optical pulses. The radiation from the laser is modulated in accordance with the signals to be transmitted. In this way, a series of optical pulses is formed which can be transported to an optical receiver via an optical transmission medium, for example an optical fiber, in which receiver they are reconverted to a digital electric signal. In such systems the aim is, inter alia longer transmission distances. However, this is impeded by a number of factors.
A first factor is that the width of the optical pulses propagating in the transmission medium increases as the length of the transmission medium increases. This pulse widening is due to the fact that the delay time in the transmission medium is different for radiation of different wavelengths. This phenomenon is referred to as dispersion. A pulse transmitted by an optical transmitter will generally comprise components of different wavelengths which arrive at the receiver at different instants due to the dispersion.
Another factor impeding the realisation of long transmission distances is the inaccurate definition of the pulse position with respect to the pulse period. This phenomenon is referred to as time jitter. It is caused by the fact that instabilities in the transmission medium will lead to a variation in the position of the signal pulses along the time axis. On the other hand, the clock signal, in other words the repetition time with which the arriving signal pulses are detected in the receiver, is a constant. In fact, the clock signal is determined as a mean value of a number of pulse periods of the signal pulse series. Due to the spread of the signal pulses with respect to time, caused by line instabilities, the relevant signal pulse and a pulse of the clock signal will not coincide and give rise to erroneous information in the receiver.
A third factor is variable damping. Due to disturbances in the transmission medium, variations in the pulse amplitudes of the signal pulse series will occur.
To reduce the detrimental effect of the above-mentioned factors on the transmission distance, a number of locations in the transmission medium may be provided with an optical unit for regenerating signal pulses.
Also the receiver itself may be provided with such a unit so as to regenerate the arrived signal pulses before these pulses are transmitted to the detection channels.
An optical unit for regenerating signal pulses of the type described in the opening paragraph is known, for example from the article "All-optical regenerator based on non-linear fiber Sagnac interferometer" by M. Jinno and M. Abe in Electronics Letters, July 1992, vol. 28, no. 14, pp. 1350-1352. The unit described in this article comprises an optical switch in the form of a known NOLM (non-linear optical loop mirror). The clock signal is applied to the input of the switch and split into two substantially equal sub-signals by a 50:50 coupler. The two sub-signals will traverse the ring in opposite directions. The signal pulse series is coupled into the ring itself as a control pulse series. The switch is open whenever a pulse of the signal pulse series, i.e. a "1" in the digital signal, travels in the ring along with the part of the clock signal propagating in that direction. During this period there will be such a phase difference between the two parts of the clock signal propagating in opposite directions that there will be constructive interference at the output of the switch. The pulses which are formed at the output of the switch thus have the intensity and the wavelength of the pulses of the clock signal.
A drawback of the known unit is that the signal pulses must be considerably amplified before they are injected into the ring of the optical switch and/or the length of the optical fiber of the ring must be quite large to reach a sufficient switching power, in other words to cause a sufficiently large phase difference between the two sub-signals of the clock signal propagating in opposite directions. In fact, the phase difference between the two sub-signals propagating in opposite directions is given by .DELTA..delta.=2.pi..n.sub.2.L.I, in which n.sub.2 is the non-linear coefficient of the refractive index of the optical fiber, L is the length of the optical fiber and I is the optical intensity of the signal pulse travelling along in the ring.
Such drawback is less predominant if use is made of, for example a known SLALOM (semiconductor laser amplifier optical mirror) for the optical switch, because the non-linearity of the amplifier instead of that of the optical fiber is used in this case.
However, a further considerable drawback occurring both in a NOLM and in a SLALOM as optical switches is that, whenever a regeneration has taken place, that changes the wavelength of the signal pulse series. The transmission system as a whole will therefore be more complicated because it is no longer sufficient to tune the complete system to a single wavelength. In fact, the pulse series supplied by the regenerating unit has taken over determination of the wavelength of the clock signal. This is undesirable, particularly for multiplex transmission systems, in which different signals are combined in one and the same channel and in which wavelength demultiplexing is used in the receiver.